Conventionally, multilayer structures including a layer made of polyolefin, such as polyethylene and polypropylene, and a layer made of a saponified ethylene-vinyl acetate copolymer (hereinafter, may be abbreviated as an EVOH) excellent in barrier properties are widely used for various applications, particularly such as food packaging containers and medicine containers, taking advantage of their barrier properties. Such a multilayer structure is used as various molded products, such as a film, a sheet, a cup, a tray, and a bottle. At this time, there is a case that offcuts, defective products, and the like generated when obtaining the various molded products are collected and melt molded to be reused as at least one layer of a multilayer structure including a polyolefin layer and an EVOH layer. Such a recovery technique is useful from the perspective of the reduction of wastes and the economic efficiency and is employed widely.
However, when reusing a recovery of a multilayer structure including a polyolefin layer and an EVOH layer, gelation occurs due to thermal degradation during the melt molding or a degraded material sticks inside an extruder, and thus it used to be difficult to carry out continuous melt molding for a long period of time. Further, since such a degraded material is often mixed into a molded product, there used to be problems of generating a fish eye and generating a phase separation foreign matter (die build-up) in a molded product thus obtained. There also used to be a problem of deteriorating the appearance due to the poor compatibility of the polyolefin and the EVOH. Food and medicine packaging containers often require processing, such as disinfection and sterilization, for the contents. However, since an EVOH has decreased gas barrier properties in conditions of high temperature and high humidity, there is a demand for a multilayer structure excellent in the gas barrier properties even after going through a high temperature and high humidity state during retort processing.
As a measure to solve such problems, Patent Document 1 describes a melt molding method of blending a resin composition, having a metal salt of fatty acid and/or hydrotalcite blended into an ethylene-vinyl acetate copolymer or a saponification product thereof, into a recovery of a layered product having an EVOH layer. According to this method, it is considered that long run moldability becomes good and generation of a deposit can be suppressed.
Patent Document 2 describes a resin composition made of polyolefin; a saponified ethylene-vinyl acetate copolymer having an ethylene content ratio of from 20 to 65 mol % and having vinyl acetate components with a degree of saponification of 96 mol % or more; at least one type of compound selected from a metal salt of higher fatty acid having a carbon number of from 8 to 22, a metal salt of ethylenediaminetetraacetic acid, and hydrotalcite; and a saponified ethylene-vinyl acetate copolymer having a percentage of ethylene content of from 68 to 98 mol % and having vinyl acetate components with a degree of saponification of 20% or more. It is considered that this resin composition has excellent compatibility, a molded object obtained using this resin composition has a surface with no wave pattern generated therein, and it has a good appearance.
Patent Document 3 describes a multilayer structure having a layer made of a resin composition having an ethylene-vinyl acetate copolymer and a saponified ethylene-vinyl acetate copolymer, having an ethylene content of 70 mol % or more, blended into a recovery of a layered product including a polyolefin-based resin layer and an EVOH layer. It is considered that, in this multilayer structure, a deposit and discoloration are suppressed to be excellent in appearance.
However, the inventions described in Patent Documents 1 through 3 used to have problems of a molded object thus obtained having insufficient gas barrier properties after retort processing and further possibly causing a defective appearance.
Patent Document 4 describes a layered product having an EVOH layer as an intermediate layer and made of a layer of a resin composition, containing a polyolefin-based resin, an EVOH, and a styrene-based thermoplastic elastomer via an adhesive layer, and polyolefin-based resin layers as inner/outer-most layers. It is considered that this layered product has good gas barrier properties in conditions of high temperature and high humidity, such as retort processing. However, since a price for a styrene-based elastomer is expensive as a resin, it is sometimes out of accordance with the economic efficiency.
Further, Patent Document 5 describes a multilayer structure having an EVOH layer as a gas barrier layer, arranged therein, unsaturated carboxylic acid-modified polyolefin layers on both sides thereof, amorphous polyamide layers on both sides thereof, resin composition layers on both sides thereof containing resins used for the EVOH layer, the propylene-based polymer layers, the unsaturated carboxylic acid-modified polyolefin layers, and the amorphous polyamide layers, and further propylene-based polymer layers on both sides thereof. It is described that the resin composition layers can be formed by reusing scraps generated when obtaining a molded object and a multilayer structure thus obtained has good gas barrier properties in conditions of high temperature and high humidity before and after retort processing. However, since it has a layer constitution of nine layers, molding devices are limited.